German Patent No. 44 18 112 implies that, particularly because of the reduction in fuel consumption, it is known to operate internal combustion engines at a stoichiometric air-fuel ratio (.lambda.=1) only when a high output is desired--in the upper load range and at full load of the engine--however in the case of lower load and idling, to adjust a lean air-fuel ratio.
During lean-burn operation, the fuel is burned with an excess of air. However, with increasing excess air, the ignitability of the fuel-air mixture deteriorates. To assure reliable ignition of the lean mixture, devices are used which generate increased movement of the fuel-air mixture in the combustion chamber of the engine, and consequently improve the flammability. Such a device is known, for example, from European Patent No. 112 494 or German Patent No. 36 19 956. It is a device by which the valves of the combustion chamber are variably controllable, i.e., a turbulence of the mixture in the combustion chamber can be produced by a change in the valve positions.
The device for improving the movement of the fuel-air mixture in the engine combustion chamber is always activated when a switchover is made to lean-burn operation, and deactivated when there is a change from lean-burn operation to stoichiometric operation. Thus, there is an operating mode in which an intensified movement of the fuel-air mixture is taking place, and an operating mode in which the movement of the mixture is less. During the switchover between the two modes, mixture inhomogeneities occur which influence the output torque of the engine, which in turn means that the vehicle driving performance is impaired.